Also, evolutionary pressure tends to make viruses less deadly. Variants that don't kill or incapacitate the host can spread faster, since the carriers are up and moving around. They'll inoculate the population against more deadly variants in the process. The longer a virus hangs around, the more variants there will be--with the more lethal ones tending to fade away, and the less lethal ones spreading far and wide.
The "Genomic Lineages" graph of Covid variants from the UKHSA gives a good view of how variants follow on from one another, at least with Covid. Flu certainly had three (?) popular variants doing the rounds (one of which was thought to have been eliminated by lockdown) but at the moment, it seems like there's usually one Covid to rule them all at any one time:
This is true for flu viruses, not covid, as it spreads before symptoms show, so there is no pressure on it to become less deadly, as the fate of the host is immaterial.
Of course there is still pressure on it to become less deadly. If deadly virus starts spreading, no matter when the symptoms show, people will start washing hands, wearing masks and socialize less. All those acts put pressure on virus to become less deadly.
Previous poster was implying that such measures will spread with any "sufficiently deadly" virus, and that presumably COVID19 is not perceived as such at this time (rightly or wrongly).
